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 Insects that undergo complete metamorphosis have four distinct life cycle stages: egg, larva, pupa, and adult.  During the insect’s larva stage, it grows and stores up energy needed for the next developmental stage (pupa), by eating large quantities of food.  As an insect enters its pupa stage, it forms a harder outer shell for protection. The insect will remain as still as possible, in order to use its energy toward the transformation process into an adult.  The adult insects no longer grow in size, but focus their energy on gathering food and reproduction. C OMPLETE M ETAMORPHOSIS

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I NCOMPLETE M ETAMORPHOSIS  Insects that undergo incomplete metamorphosis have three distinct life cycle stages: egg, nymph, and adult.  Nymphs often closely resemble the adult, but are smaller in size and lack wings.  Nymphs shed their exoskeleton as they grow, and do not develop wings until they are adults.  Adult insects focus their energy on survival and reproduction.

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H ELPFUL VS. HARMFUL  Insects can be very helpful to humans:  The honey bee helps pollinate crops, so we have fruits and vegetables to eat.  Beetles and flies are decomposers, keeping ecosystems healthy and adding nitrogen back into the soil.  Many food chains and food webs depend on insects.

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HELPFUL VS. HARMFUL  Insects can be very harmful:  Some insects carry diseases, such as malaria (mosquitoes) and plague (fleas).  Some insects destroy crops, such as locusts or boll weevils.  Some invade our homes, such as cockroaches.

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I NSECT C ONTROL  Most of the time, insects are no problem.  Occasionally, we need to control the population.  Humans have learned to use chemicals, called insecticides and pesticides, to kill insects.  Sometimes, these chemicals stay in the environment and cause some unintended consequences.

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U NINTENDED C ONSEQUENCES  In the 1960’s, scientists noticed many birds of prey were not able to hatch their young. The shells of the eggs were too fragile for the parent to sit on without breaking them.  Scientists performed investigations and tracked the problem to a pesticide called DDT.  Farmers had sprayed their crops with DDT to kill insects, such as the boll weevil.

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U NINTENDED C ONSEQUENCES  When it rained, the pesticide would wash off the plants and flow in run-off water into streams and other areas where water accumulated.  The pesticide got into the bodies of small fish, which in turn, were eaten by larger fish. Eventually birds, such as the eagle and the brown pelican, ate the large fish.  When DDT was transferred into the birds, the chemical made the shells of the eggs easy to break (fragile). There was a dramatic drop in eagle and brown pelican populations.

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E COSYSTEMS  Learning about insecticides and pesticides helped us to realize that everything in the ecosystem is connected.  Human activity can have an impact on the environment.